Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
The accuracy and precision of many therapeutic imaging interventions is challenged by respiratory motion. Ventilation-induced tumor and organ-at-risk oscillations are of concern primarily for imaging lung and upper abdominal malignancies. Four dimensional (4D) computerized tomography (CT) is currently the most widely adopted modality for imaging organs that are subject to respiratory motion. However, the mechanical design of CT scanners limits the sampling pattern and often leads to resampling artifacts. This modality also provides poor soft tissue contrast.
Magnetic resonance imaging (MRI) can provide improved soft tissue contrast relative to CT. Further, the sampling function of an MR imager can be easily modified and adapted to, e.g., facilitate imaging of particular portions of anatomy, to detect a flow tensor or other specialized properties of tissue, to emphasize imaging of certain types of tissue or contrast agents, or to provide some other benefit. However, MRI can impose strict timing requirements when imaging tissue, requiring extensive time for acquisition and/or numerous MR scans to generate an adequate image of tissue. These factors have limited application of MRI to the imaging of tissues that are in motion, for example, the lungs, heart, and other tissues of the abdominal and/or thoracic cavities during respiration.